A wet carbon dioxide environment having a low H.sub.2 S concentration is frequently encountered by materials used in the fields of extraction, transport, refinement, storage, and the like of oil and natural gas, and is thus a significant environment for these fields of industry. The energy industry, the chemical synthesis industry, and the like which support modern society depend on crude oil, natural gas, and the like. Also, crude oil and natural gas are difficult to secure or utilize without materials that are resistant to the environment. Accordingly, it is not too much to say that materials resistant to the environment support modern society.
For welded steel structures used in the environment, most significant problems to solve are sulfide stress cracking (hereinafter referred to as SSC) in the heat affected zone (hereinafter referred to as HAZ) of a steel and corrosion resistance of a weld metal. SSC in HAZ refers to occurrence of SSC in HAZ hardened by welding. Herein, SSC refers to hydrogen-induced cracking, i.e. cracking induced by entry of hydrogen generated by a cathodic reaction into steel, and is distinguished from stress corrosion cracking (hereinafter referred to as SCC) accompanied by an anodic reaction accompanied with solution of electrode.
Corrosion of a weld metal refers to both "corrosion induced by trace hydrogen sulfide" contained in wet carbon dioxide gas and "corrosion induced by wet carbon dioxide gas". A type of corrosion corresponds to both general corrosion and pitting corrosion of a weld metal. Corrosion induced by hydrogen sulfide is called "hydrogen sulfide corrosion." Particularly, corrosion emerging in the form of cracking and accompanied by an anodic reaction making the electrode dissolve is called SCC.
As will be described later, welded steel structures, for example, welded piping, endurable of long-term use in the environment are so far limited to those manufactured of expensive materials. Accordingly, in the above-mentioned oil extraction and like industries there has been eager demand for inexpensive welded steel structures having SSC resistance and corrosion resistance.
As disclosed by the inventors of the present invention in Japanese Patent Application Laid-Open (kokai) No. 5-287455, low carbon stainless steel containing Mo and 13% Cr exhibits sufficient corrosion resistance in a wet carbon dioxide environment with a low H.sub.2 S concentration and is used by many users as material for an oil well pipe. That is, the low carbon stainless steel containing Mo and 13% Cr raises no problem as material for an oil well pipe unless welding is involved.
However, when a welded steel structure composed of the low carbon stainless steel containing Mo and 13% Cr serving as a base metal and a steel having the same chemical composition as that of the base metal serving as a welding material is used in a wet carbon dioxide environment with a low H.sub.2 S concentration, SSC occurs in HAZ, and a weld metal exhibits a relatively high corrosion rate. Thus, the welded steel structure fails to endure the use in the environment.
By contrast, a welded steel structure employing Cr-concentration-increased duplex stainless steel containing 22% Cr as both a base metal and a weld metal exhibits excellent SSC resistance and corrosion resistance in the aforementioned environment (M. Ueda et al; "Performance of high corrosion resistant duplex stainless steel in chloride and sour environments, Corrosion/93, Paper No. 125, NACE International, Houston (1993)).
Accordingly, in a wet carbon dioxide environment with a low H.sub.2 S concentration there is used a welded steel structure composed of a base metal formed from duplex stainless steel containing 22% Cr and a weld metal formed from duplex stainless steel containing 22% Cr. Particularly, welded piping using seamless pipe as a base metal is used in such an environment.
However, duplex stainless steel containing 22% Cr is more expensive and lower in strength than is stainless steel containing 13% Cr.